The US breached international law when Texas executed a Mexican citizen convicted of rape and murder, the UN claimed last night, because he was not given access to consular officials.

Governor Rick Perry of Texas, who is widely expected to enter the race for the Republican presidential nomination within weeks, declined to grant a last-minute 30-day reprieve to Humberto Leal Garcia, 38, sending the killer to his death despite an appeal from President Barack Obama to spare him.

Navi Pillay, the United Nations High Commissioner for Human Rights, said: "The execution of Mr Leal Garcia places the US in breach of international law. What the state of Texas has done in this case is imputable in law to the US and engages the United States' international responsibility."

Mexico said that Mr Perry refused to take a telephone call from its ambassador to the United States as the death chamber was being prepared. He had the power to grant a one-off 30-day stay of execution but not to pardon or commute the sentence without a recommendation from the Texas Board of Pardon and Paroles, which voted against sparing Leal.

An hour before the execution, the Supreme Court voted by five to four to allow the death by lethal injection to go ahead despite arguments from the Obama administration, Mexico and the UN that it was in breach of the Vienna Convention.

Leal was found guilty of the murder of a girl of 16 whose brutalised body, the head smashed in with an asphalt block, was found in 1994.

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The Obama administration, and the Bush administration before it, argued that executing Leal would endanger American citizens abroad because other countries might be encouraged to deny them consular assistance.

Leal was an illegal immigrant who moved to the United States when he was a toddler. The state of Texas argued that Leal did not tell police he was a Mexican citizen and the issue on consular access was not raised during his trial.

Before the injection was administrated in the death chamber in Huntsville, Texas, the condemned man repeatedly apologised.

He twice shouted "Viva Mexico!" before telling the prison warden he was ready and adding: "Let's get this show on the road."

Texas executes more prisoners than any other US state. In his 10 years in office Mr Perry has refused clemency in 231 executions, almost half the 470 executions in Texas since the death penalty was reinstated in 1974.